Facebook is looking at charging users for sending e-mail messages to other users who are not in their online social circle, as it looks for alternative revenue sources besides advertising.

In a blog post issued Thursday, the social networking giant said it is starting a "small experiment" to test the usefulness of "economic signals" to determine the relevance of messages sent to users. This trial would give a small number of people the option to pay to have a message routed to the Inbox folder of a recipient they are not connected to, which would otherwise have landed in the person's Other folder, it explained.

"Several commentators and researchers have noted that imposing a financial cost on the sender may be the most effective way to discourage unwanted messages and facilitate delivery of messages that are relevant and useful," Facebook said.

The company currently relies on social "signals", such as friend connections on the platform to determine whether a message is likely to be one users wants to to see in their Inbox. It also depends on algorithms to identify spam and use broader signals from the social graph--such as friend of friend connections or people the user may know--to determine relevance, it explained.

The trial is currently open to select users in the United States, and the number of messages a person can have routed from their Other folder to the Inbox folder will be limited to one per week, it added.